From U.S. Pat. No. 3,808,995 a sewing machine is known having a lower feed dog and an upper feed dog which carry out a transport movement in a feed direction. The transport movement is adjustable separately for the lower feed dog and for the upper feed dog. By the combination of a lower feed dog with an upper feed dog a multiple-ply workpiece can be moved without displacing the individual plies relative to each other in an undesired manner. A desired relative motion, on the other hand, can be carried out.
If arched or corner type seams are to be made with the known sewing machine, it is necessary, because of the feed direction of the feed dog and upper feed dog is invariable as to angle, to turn the workpiece accordingly. This requires reducing the sewing speed or interrupting the sewing operation, whereby the production times and hence the costs per workpiece are increased. This is true in particular for large-area workpieces, as they are especially cumbersome to handle.
From U.S. Pat. No. 3,055,325, a sewing machine is known which comprises a lower feed dog acting in normal feed direction as well as in transverse feed direction. To initiate the movement in transverse feed direction, the movement taken off from a lower machine shaft is transmitted via an eccentric drive to an oscillating shaft and from the latter via a gear arrangement to a feed dog holder on which the feed dog is mounted. The amplitude of oscillation of the oscillating shaft can be changed either by a stitch setter or, to obtain a given seam pattern, by a cam.
With the arrangement according to U.S. Pat. No. 3,055,325, it is possible to obtain an overstitch enlarged in relation to the sideways swinging movement of the needle. Further it is possible to move the work either exclusively in the feed direction or exclusively in the transverse feed direction, in which case the respective stitch setter must be brought into the zero position. If both stitch setters adjustable by a common handle occupy a position differing from their zero position, the work can be transported in any desired direction.
In sewing machines of the above-described kind with a lower feed dog the disadvantage appears that plies of a workpiece which are to be sewn in equal lengths execute an undesired relative motion due to the difference in the speeds between the lower ply, moved by the feed dog, and the upper ply, frictionally taken along by the latter but decelerated by the braking action of a presser foot. This disadvantage, known in the sewing practice, is felt more strongly with a sewing machine which transports in any direction (i.e. including directions which deviate angularly from the longitudinal direction in the feed plain, because the error caused thereby now occurs not only along a line but the error is noticeable over the entire surface of this ply in accordance with the seam course.